The Cardiovascular System - Workings: how the cardiovascular system
functions

Photo by: Sebastian Kaulitzki

In its continuous work, an average heart contracts more than 100,000
times a day to force blood through the thousands of miles of blood
vessels to nourish each of the trillions of cells in the body. With each
contraction, the heart forces about 2.5 ounces (74 milliliters) of blood
into the bloodstream. At an average adult heart rate of 72 beats per
minute, this equals about 1.4 gallons (5.3 liters) of blood every
minute, 84 gallons (318 liters) every hour, and 2,016 gallons (7,631
liters) every day. With exercise, this amount may be increased by as
much as five times.

Cardiac cycle

Cardiac cycle refers to the series of events that occur in the heart
during one complete heartbeat. Each cardiac cycle takes about 0.8
second. During this brief moment, blood enters the heart, passes from
chamber to chamber, then is pumped out to all areas of the body. Each
cardiac cycle is divided into two phases. The two atria contract while
the two ventricles relax. Then, the two ventricles contract while the
two atria relax. The contraction phase, especially of the ventricles, is
known as systole; the relaxation phase is known as diastole. The cardiac
cycle consists of a systole and diastole of both the atria and
ventricles.

In order to increase their endurance before competition, some athletes
resort to a technique known as
blood doping.
The procedure involves withdrawing some of the athlete's red
blood cells. After the blood is removed, the athlete's body
responds by quickly producing more red blood cells to replace those
withdrawn. Then, a few days before a competitive event, the withdrawn
blood is infused back into the body.

The effect is to create a greater number of red blood cells and, in
turn, a greater concentration of oxygen in the blood. Blood doping can
increase an athlete's aerobic capacity by up to 10 percent.

However, blood doping is not only illegal but risky. It can impair
blood flow as well as cause flulike symptoms. Instead of helping an
athlete's performance, it can hinder it.

The process begins as deoxygenated (carrying very little oxygen) blood
returns to the right atrium of the heart via the venae cavae. At the
same time, oxygenated blood transported from the lungs by the four
pulmonary veins empties into the left atrium. The AV valves open, and as
blood flows into the atria it also flows passively into the ventricles.
The semilunar valves, however, are closed to prevent blood from flowing
out of the ventricles into the arteries. When the ventricles are about
70 percent full, the SA node sends out an impulse that spreads through
the atria to the AV node. The atria contract, pumping out the remaining
30 percent of blood into the ventricles.

The AV node slows the impulse briefly, allowing the atria time to
complete their contraction. The impulse then travels through the AV
bundle, the bundle branches, and the Purkinje fibers to the apex of the
heart. As the contraction of the ventricles is initiated at this spot,
pressure begins building rapidly in the ventricles and the AV valves
close (the "lub" sound heard through a stethoscope) to
prevent blood from flowing back into the atria. When the pressure in the
ventricles becomes higher than the pressure in the large arteries
leaving the heart, the semilunar valves are forced open and blood is
pumped out of the ventricles. Deoxygenated blood in the right ventricle
is pumped to the lungs via the pulmonary arteries; oxygenated blood in
the left ventricle is pumped to the rest of the body via the aorta.

While the ventricles are contracting (systole), the atria are at rest
(diastole) and are filling with blood once again. When all the blood is
pumped from the ventricles, the semilunar valves close (the
"dup" sound heard through a stethoscope) to prevent the
backflow of blood into the heart. For a moment, the ventricles are
empty, closed chambers. When the pressure in the atria increases above
that in the ventricles, the AV valves are forced open and blood begins
to flow into the ventricles, starting a new cardiac cycle that will take
less than one second to complete.

In short, during the cardiac cycle, the upper half of the heart (the
atria) receives blood. The lower half (the ventricles) then pumps out
the blood. The right side of the heart (right atrium and right
ventricle) receives and pumps out deoxygenated blood; the left side
(left atrium and left ventricle) receives and pumps out oxygenated
blood.

Blood pressure

When the ventricles contract, they force or propel blood from the heart
into the large, elastic arteries that expand as the blood is pushed
through them. The pressure the blood exerts against the inner walls of
the blood vessels is known as blood pressure. This pressure is necessary
to keep the blood flowing to all areas of the body and then back to the
heart.

Blood pressure is greatest in the large arteries closest to the heart.
Because their walls are elastic, the arteries are able to recoil and
keep most of the pressure on the blood as it flows away from the heart.
As the blood courses through the system in less elastic
vessels—arterioles into capillaries into venules into
veins—blood pressure drops. When the blood finally returns to the
right atrium via the venae cavae, the pressure behind it is almost zero.

Since the heart contracts and relaxes during a cardiac cycle, blood
pressure rises and falls during each beat. It is higher during systole
(left ventricle contracting) and lower during diastole (left ventricle
relaxing).

Blood pressure is measured in millimeters of mercury (mmHg) with a
sphygmomanometer (see box). A blood pressure reading is most often taken
on the brachial artery in the arm. The systolic pressure is recorded
first, followed by the diastolic pressure. Average young adults have a
blood pressure reading of about 120 mmHg for systolic pressure and 80
mmHg for diastolic pressure (written as 120/80 and read as
"one-twenty over eighty"). Depending on age, sex, weight,
and other factors, normal blood pressure can range from 90 to 135 mmHg
for the systolic pressure and 60 to 85 mmHg for the diastolic pressure.
Blood pressure normally increases with age.

Regulating the heart rate

Under normal circumstances, the heart controls the rate at which it
contracts or beats. But another body system—the nervous
system—can and does affect heart rate to help the body adapt to
different situations.

Medical personnel measure a person's blood pressure using an
instrument called a sphygmomanometer. This device consists of a rubber
cuff, a hand bulb pump, and a pressure-reading mechanism.

The rubber cuff of the sphygmomanometer is wrapped snugly around a
patient's arm just above the elbow. The individual taking the
blood pressure then places a stethoscope (a hearing device) against
the patient's brachial artery on the inside of the arm just
above the elbow to listen for the pulsing of the blood.

The cuff is then inflated using the hand bulb pump until the blood
flow into the arm is stopped and a pulse cannot be heard or felt. The
pressure in the cuff is then released slowly. When a small amount of
blood begins to spurt through the constricted artery, soft tapping
sounds are heard through the stethoscope. The cuff pressure reading at
which the first sound is heard is recorded as the systolic pressure.

As the pressure in the cuff is released further, the tapping sounds
become louder, then soon soften. When the artery is no longer
constricted and blood flows freely, the sounds disappear. The cuff
pressure reading at the last sound heard is recorded as the diastolic
pressure.

The medulla oblongata is a mass of nerve tissue at the top of the spinal
cord and at the base of the brain that controls involuntary processes
such as breathing and heart rate. Inside the medulla are two cardiac
centers, the accelerator center and the inhibitory center. These centers
send nerve impulses to the heart to regulate its beating.

The autonomic nervous system is a division of the nervous system that
affects internal organs such as the heart, lungs, stomach, and liver. It
functions involuntarily, meaning the processes it controls occur without
conscious effort on the part of an individual. The autonomic nervous
system is divided into two parts, the parasympathetic and sympathetic
systems. The parasympathetic system is active primarily in normal,
restful situations; the sympathetic system is most active during times
of stress or when the body needs energy.

The accelerator center in the medulla sends impulses along sympathetic
nerves to the heart to increase heart rate and the force of contraction.
The inhibitory center sends impulses along parasympathetic nerves to the
heart to decrease heart rate. The centers act in response to changes in
blood pressure and the level of oxygen in the blood, often brought about
by factors such as exercise, increased body temperature, and emotional
stress. Such changes are detected by receptors located in the carotid
arteries and the aortic arch.

Receptors in the carotid arteries detect a decrease in blood pressure;
those in the aortic arch detect a decrease in the level of oxygen in the
blood. Both send out impulses along sensory nerves to the accelerator
center, which in turn sends impulses along nerves to the SA node of the
heart to increase heart rate. When blood pressure or blood oxygen level
has been restored to normal, the inhibitory center sends out impulses
along nerves to the SA node to slow heart rate to a normal resting pace.

Exchanges between capillaries and general body tissues

Arteries, arterioles, venules, and veins: the only function of these
vessels is to transport blood from or to the heart. The exchange of
materials—oxygen, carbon dioxide, nutrients, and
wastes—between the blood and interstitial fluid occurs through
the capillaries. The movement of these materials is variously brought
about by three processes: diffusion, filtration, and osmosis.

Diffusion is the movement of molecules from an area of greater
concentration (existing in greater numbers) to an area of lesser
concentration (existing in lesser numbers). Diffusion takes place
because molecules have free energy, meaning they are always in motion.
This is the case especially with molecules in a gas, which move quicker
than those in a solid or liquid. Oxygen and carbon dioxide, the gases
that pass between the capillaries and the interstitial fluid, move by
diffusion. As blood courses through a capillary, the oxygen carried by
the hemoglobin in red blood cells exists in a greater amount and thus
moves into the surrounding interstitial fluid to be taken up by the
cells. Conversely, carbon dioxide exists in a greater amount in the
interstitial fluid and so moves into the capillary to be carried away.
This exchange of gases between the blood and the interstitial fluid is
called internal respiration.

A chart illustrating the process of diffusion. Diffusion is the
movement of molecules from an area of greater concentration to an
area of lesser concentration. (Illustration by

Hans & Cassady

.)

Filtration is the movement of water and dissolved materials through a
membrane from an area of higher pressure to an area of lower pressure.
When blood enters capillaries, it has a pressure reading of about 33
mmHg; the pressure of the interstitial fluid is only about 2 mmHg. Thus,
through filtration, plasma and nutrients such as amino acids, glucose,
and vitamins are forced through the capillary walls into the surrounding
interstitial fluid.

Osmosis is the diffusion of water through a semipermeable membrane (a
membrane that allows some materials but not others to flow through it).
It is the movement of water from an area where it is abundant to an area
where it is scarce or less abundant. Directly related to this is osmotic
pressure, which is the tendency of a solution to "pull"
water into it. The strength of this pressure is determined by the amount
of dissolved material, called solutes, in the solution. The greater the
amount of solutes, the lower the amount of water in that solution. A
solution containing a high amount of solutes has a high osmotic
pressure, and water has a greater tendency to move into the solution.

At the venous end of capillaries, just before they merge to form
venules, the osmotic pressure is greater in the capillaries than in the
interstitial fluid. This is due to the presence of albumin and other
large proteins that have

A magnified view of osmosis, the movement of water from an area
where it is abundant to an area where it is scarce or less abundant.
Osmosis plays a major role in the chemistry of living things.
(Illustration by

Hans & Cassady

.)

remained as solutes in the blood. Interstitial fluid has a low osmotic
pressure and is thus "pulled" into the capillaries and
carried away.

Capillary exchange in the lungs

After blood has flowed through the tissues of the body, exchanging
oxygen and nutrients for carbon dioxide and wastes, it heads back to the
heart. The deoxygenated blood empties into the right atrium via the
venae cavae, then into the right ventricle. From here it is pumped into
the pulmonary trunk, which then divides into the right and left
pulmonary arteries. These arteries transport the deoxygenated blood to
each lung.

In the lungs, the arteries branch out into successively smaller arteries
and successively smaller arterioles. Finally, the smallest arterioles
branch into capillaries. These pulmonary capillaries surround the
alveoli, the air sacs of the lungs. The exchange of oxygen and carbon
dioxide in the lungs, known as external respiration, takes place across
the walls of the alveoli and nearby capillaries.

As in internal respiration, the exchange of gases in external
respiration occurs according to the process of diffusion. Air in the
alveoli has a high concentration of oxygen. The blood in the pulmonary
capillaries has a high concentration of carbon dioxide. Following
diffusion, oxygen in the alveoli moves into the capillaries while carbon
dioxide in the capillaries moves into the alveoli.

Now oxygenated, blood flows from the capillaries into venules, which
merge to form larger and larger veins. Finally, the blood exits each
lung through two large pulmonary veins and is carried to the left atrium
to be pumped back into the systemic circulation once again. The movement
of blood from the lungs to the heart is a special occurrence in the
body: it is the only time that veins carry oxygenated blood.

Hepatic portal circulation

Another unique circulation route is the hepatic portal circulation, a
subdivision of the systemic circulation. Under this circulation pathway,
blood from the digestive organs and the spleen flow through the liver
before heading to the heart.

Capillaries that drain the stomach, small intestine, colon, pancreas,
and spleen flow into two large veins, the superior mesenteric and the
splenic. These two veins then unite to form the portal vein, which
carries the blood into the liver.

Once in the liver, the portal vein branches to form capillaries called
sinusoids. Sinusoids are larger than normal capillaries. Their walls are
also more permeable, allowing proteins and blood cells to enter or leave
easily. This is important since the blood entering the liver from the
digestive organs contains large amounts of nutrients.

As the blood flows slowly through the sinusoids in the liver, some of
these nutrients are removed from the blood and either stored in the
liver for later use or changed into other materials the body needs. From
the sinusoids, blood flows into the right and left hepatic veins, then
into the inferior vena cava, and finally into the right atrium.

The complete flow of blood from the digestive organs to the heart is
unusual. Normally, arteries flow into capillaries, which flow into
veins. In the hepatic portal circulation, no arteries are involved.
Here, capillaries merge to form veins, which branch into capillaries
that merge again to form veins. This strange route is necessary so that
blood may be altered by the liver. Nutrients may be stored or changed
and possible poisons (such as alcohol and medicines) may be transformed
into less harmful substances before the blood returns to the heart and
the rest of circulation.

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